At present, a crane mounted on a working machine is in most cases still controlled by controlling the actuator provided in connection with each boom of the crane separately. As a result, the driver has to combine the movements of various actuators in order to make the crane move in a desired way. Normally, cranes are used for moving an implement fastened to the head of the crane. For example in a forest machine, the implement is a harvester head or a loading grapple, depending on whether the machine is a harvester or a forwarder. Largely depending on this are also the most typical movements that are usually implemented with the crane when working with the working machine. In a forwarder used for loading of timber, the crane is normally moved from the load space of the forwarder to the side of a log pile and back to the load space. When a harvester is used for felling of timber, the crane is normally moved in the horizontal direction from the working machine towards the trees to be felled and, after the felling, in a suitable direction where a pile of cut and delimbed tree trunks is placed. Normally, however, in all working machines, also other than forest machines, the aim of the movements of the crane is to move the implement at the head of the crane from one place to another. This may be the reason why a method has been recently developed for controlling the crane in a way that serves this aim better and thereby facilitates the work of the driver of the machine. In this so-called boom tip control (or coordinated control), the driver controls the crane in such a way that the control devices of the working machine are used for directly controlling the movement of the head of the crane in different directions of motion. Thus, a single control function of the control device is used for controlling the head of the crane to move e.g. in the horizontal direction away from the working machine and back towards the working machine, another control function is used for controlling the movement upwards and downwards in the vertical direction, and a third control function is used for controlling the turning of the whole crane to the left and to the right in the horizontal direction. For the driver, this so-called boom tip control is motorically less demanding, because when it is applied, the driver does not need to control the different actuators separately and to know how to combine the relationship between the movements of the single booms generated by them with the movements of the head of the crane, but this is performed automatically by the control system of the working machine according to the direction of motion where the driver wants the head of the crane to move. As a result, boom tip control has been found to make the work easier and more efficient, particularly for a driver with little experience. Such a method and apparatus for controlling the crane on the basis of boom tip control is described in the thesis by Markus Saarela, “Coordinated Motion Control of a Log Loader Boom”.
Normally, the crane comprises two to three booms connected to each other and to the working machine in an articulated or otherwise movable manner. For example, the forest machines manufactured by the applicant typically comprise a hoisting boom connected at its first end in an articulated manner to a traverser mounted on the body of the working machine in a swiveled manner with respect to the vertical axis, a stick boom connected at its first end in an articulated manner to the second end of the hoisting boom, as well as an extension boom movable linearly out of the second end of the stick boom, in the longitudinal direction of the stick boom, and back into the stick boom. Now, if e.g. such a crane is controlled with boom tip control, the control system has, in principle, an infinite number of alternatives selectable for implementing a movement in a given direction of motion. Depending on where the head of the boom is moving with respect to its range of motion, some of these alternatives are always such that their implementation will lead to a situation in which the movement of the crane is more difficult than the implementation of another alternative, because of one or more factors limiting the movement of the booms of the crane. These limitations caused by the principle of operation of the crane are not taken into account in tip-control-based control methods of prior art; therefore, the cranes of prior art operated by boom tip control cannot always move the head of the crane in a desired way. This may mean, for example, that in a certain position the head of the crane will not move at all if an attempt is made to steer it in a direction whose implementation brings the control system to a situation in which it would have to move an actuator although this is, for example, in its extreme position, or in which a boom would hit the working machine, the ground, or another surrounding obstacle. In such a situation, the driver has to move the crane first in a suitable direction, in which the boom tip control can move the crane, to be able to continue the work.